what(): basic_string::_M_construct null not valid

Question

I am making a program where I need to use a function which stores a tokens of a string in a vector. The function did not work properly so I tried the function on a smaller program. Of course, I used string tokenizer function. But it is not working as expected. First, here's the code:

#include <vector> #include <string> #include <cstring> using namespace std; int main() { vector<string> v; string input = "My name is Aman Kumar"; char* ptr = strtok((char*)input.c_str(), " "); v.push_back((string)ptr); while(ptr) { ptr = strtok(NULL," "); v.push_back((string)ptr); } cout<<"Coming out"; for(string s:v) { cout<<s<<endl; } }

Now the problems. I think the issue has something to do with the command:

(string)ptr

This thing works perfectly in the first call, but gives error when present in the while loop. If I comment it out and print ptr, then it works okay, but then the program terminates after the while loop, and doesn't even execute

cout<<"coming out";

leave alone the contents of the vector. But again, if I don't print ptr too, then the first Token "My" which was stored in the vector gets printed. I literally cannot find what is causing this. Any suggestion would be helpful.

It seems you are invoking string constructor with null pointer which is not allowed. — dewaffled
– dewaffled, Commented Jun 25, 2020 at 8:32
Does this answer your question? Assign a nullptr to a std::string is safe? — dewaffled
– dewaffled, Commented Jun 25, 2020 at 8:33
char* ptr = strtok((char*)input.c_str(), " "); this could be a problem. c_str() returns const C string and you must not change it. But you are removing the constness with (char*)input.c_str() and passing it to strtok. strtok modifies the strings. That causes undefined behavior. — Thomas Sablik
– Thomas Sablik, Commented Jun 25, 2020 at 8:33
You're not allowed to modify the result of input.c_str(). Use std::string functions and <algorithm> instead of the old C interface. — molbdnilo
– molbdnilo, Commented Jun 25, 2020 at 8:35
The short and safe version: std::istringstream stream(input); std::copy(std::istream_iterator<string>(stream), std::istream_iterator<string>(), std::back_inserter(v));. — molbdnilo
– molbdnilo, Commented Jun 25, 2020 at 8:40

Alan Birtles · Accepted Answer · 2020-06-25 08:39:03Z

In

while(ptr) { ptr = strtok(NULL," "); v.push_back((string)ptr); }

For the last token ptr will be null, constructing a std::string from a null pointer is undefined behaviour. Try:

while(ptr = strtok(NULL," ")) { v.push_back(string(ptr)); }

For a more c++ solution:

#include <vector> #include <string> #include <iostream> #include <sstream> int main() { std::vector<std::string> v; std::string input = "My name is Aman Kumar"; std::stringstream ss(input); std::string word; while(ss >> word) { v.push_back(word); } std::cout << "Coming out\n"; for(std::string& s:v) { std::cout << s << "\n"; } }

Thanks a lot. Can you tell me why in the last for loop you have used string& instead of string? For loop surely have the same object and not a copy object, right?
@ProfessorofStupidity The vector v is the same object, but the loop variable s works the same as other variables.
@ProfessorofStupidity without the reference you would be making a copy of the strings in the vector which isn't necessary

Yksisarvinen · Accepted Answer · 2020-06-25 08:45:44Z

You don't know if ptr is nullptr before you try to create string out of it (and calling std::string construtor with nullptr is UB)

You need to reorganize your loop, e.g. like this:

char* ptr = strtok(input.data(), " "); while(ptr) { v.push_back(std::string(ptr)); ptr = strtok(NULL," "); }

As a side note, don't use C-style cast syntax in C++. It it very likely to hide problems, and C++ syntax offers much safer alternatives.

Casting away constness and modifying result is UB in C++, so first cast can be replaced with data call (which returns pointer to non-const when needed). If you don't have C++11, then this is UB anyway, because string was not guaranteed to store memory in continuous memory and you need to use different methods.

Casting away constness is not a problem (it would be impossible to interface with a lot of C code otherwise). It's subsequent modifications that cause undefined behaviour.

Serge Ballesta · Accepted Answer · 2020-06-25 08:59:11Z

You are mixing std::string with the C-ish strtok function. That is really a bad idea. std::string are more or less intervertible with const char * but not with mutable char *. Not speaking of the null pointer question. So choose one method and stick to it.

C-ish one: build a plain char array and store vectors of char *:

 char *cstr = strdup(input.c_str()); ptr = strtok(cstr, " "); while(ptr) { v.push_back(ptr); ptr = strtok(NULL, " "); } cout<<"Coming out"; for(char *s:v) { cout<<s<<endl; } free(cstr); // always free what has been allocated...

C++ one, use a std::stringstream:

 std::stringstream fd(input); for(;;) { std::string ptr; fd >> ptr; if (! fd) break; v.push_back(ptr); } cout<<"Coming out"; for(std::string s: v) { cout<<s<<endl; }

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what(): basic_string::_M_construct null not valid

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